The present invention relates to a wear resistant cylinder such as an Al alloy tappet in which the outer circumferential surface is coated with a hard film, and a method of manufacturing it.
A tappet body used in a direct acting type valve to operating mechanism in an internal combustion engine is generally formed from Fe material such as steel and cast iron. Recently, in order to lighten the valve operating system, it is inclined to be made of Al alloy.
However, Al alloy tappet provides lower mechanical strength and lower wear resistance than Fe material. Thus, when the tappet is inserted in a cylinder head made of the same material, Al alloy, the sliding surface is likely to wear earlier or to cause scoring.
To solve the problem, the outer circumferential surface of Al alloy tappet may be coated with wear resistant material which is different from a base metal. If the outer circumferential surface of the base metal is flat, adhesion strength of the wear resistant material is low, and high peel resistance is not available, so that it is necessary to perform preliminary treatment for the base metal to make the whole outer circumferential surface of the tappet to a rough surface.
To make the rough surface, blasting is generally applied, but for the treatment, it is necessary to employ a specialized blasting apparatus, which takes a long time to involve low productivity and high consumption of blasting material, thereby increasing manufacturing cost.
To overcome the problem, U.S. Pat. No. 5,605,102 issued to Nobuo Hara et al. discloses a tappet in which the outer circumferential surface is made to a rough surface by simple means and is coated with wear resistant material.
Describing the invention in the U.S. Patent, as shown in FIG. 10 of an attached drawing, the whole outer circumferential surface of Al alloy cylindrical tappet is made to a threaded uneven surface 2 which comprises a projection 2a and a groove 2b, and is thermally sprayed to form a film 3.
The whole outer circumferential surface of the tappet body comprises the uneven surface 2 and the film 3 thereon. The projection 2a and groove 2b provide high adhesion strength of the film 3 to the base metal to improve peel resistance in an axial direction. But, in a circumferential direction, there is neither adhesiveness nor frictional resistance by the projection and groove, so that high peel resistance is not obtained, thereby providing low durability or reliability of the tappet.